How can i measure projected area of any object in blender ? (Ex. shadow from top view)
The easiest way I know :


in orthographic top view, project the UVs from view with U > Project from View :





in the UV/Image editor, export the UV layout as a vector image by going in UVs > Export UV Layout and selecting Scalable Vector Graphic (.svg) :





in Inkscape or Illustrator or any vector program, select everything and disable the stroke to keep only the fill :





still with everything selected, do a union boolean to get a single path :





save the svg file
back in Blender, go in File > Import > Scalable Vector Graphics
with the path imported and selected, do Object > Convert to > Mesh from Curve
do Object > Transform > Origin to Geometry and Alt+G to center the object
still in orthographic top view, scale the object roughly :





in Edit mode, enable snap to vertices, select everything and snap a  vertex to its corresponding vertex on the original mesh, even if the others don't align :





select that vertex and snap the 3D cursor to it with Shift+S > Cursor to selected
select everything and make a vertex on an opposite side of the mesh active with Shift+Right Click twice :





enable Pivot to 3D Cursor and Snap to Active :





scale only horizontally with S+Shift+Z and point your mouse to the vertex corresponding to your active vertex to make it snap :





now that the mesh is perfectly aligned and scaled, go in the User Prefs and enable the MeasureIt add-on :





with everything selected in Edit Mode, go in the Tool Shelf > Display Tab, then click on Area and Show :





the area is displayed as the green circle above shows.